Threaded sucker rod joint



Jan. 13, 1970 .A. wjKAMMERER, JR 3,489,445

TI'UREADED` SUCKER ROD JOINT Filed Oct. 18. 1967 Liam? 6l smv/N 6,46 #apkesrkcss 44./00PS/ g-a/l Ps1 c le rroeA/EV.

United States Patent 3,489,445 THREADED SUCKER ROD JOINT Archer W.Kammerer, Jr., 1900 Yucca Ave., Fullerton, Calif. 92632 Filed Oct. 18,1967, Ser. No. 676,108 Int. Cl. F16b 7/18 U.S. Cl. 287--117 15 ClaimsABSTRACT OF THE DISCLOSURE A prestressed threaded joint, such as asucker rod joint used in pumping oil and similar wells and subject torepeated stress reversals during pumping of the well, comprising a pinthreaded into a companion box with a washer interposed and incompression between the pin shoulder and end of the box, the washerhaving a substantially greater modulus of elasticity than the pin andbox to significantly lower the stress added to the prestressed jointupon imposition of the pumping load thereto.

The present invention relates to threaded joints, and more particularlyto the threaded joints of sucker rods which are threadedly secured toone another to form a string of rods running between a suitable powermecha nism at the top of the well bore and a reciprocating pump in thewell bore set at a desired location therein.

In connection with deep well pumps actuated through the reciprocation ofa string of sucker rods extending from the pump to the top of the wellbore, the rods of such string are intermittently loaded upon theirupward movement as a result of the pump load imposed thereon, requiredto lift the entire height of the fluid column above the pump. Thus, acyclic or intermittent loading and unloading on the sucker rod pins andboxes occurs during operation of the well. A theory has been extant thatif the threaded joints in the string of sucker rods were initiallystressed to a degree greater than the load applied cyclically, noadditional loading would be applied on the joints when the pump load wasapplied thereto.

It has been demonstrated through actual tests that this theory ofprestressing, to avoid the addition of the deadweight load of the iiuidcolumn to the threaded pins during pumping, is in error. These testsshowed that about 4() percent of the deadweight pumping load was addedto the stress in the threaded pin in its relief section during theupstroke of the sucker rods and the pump. Such stress reversal in thesucker rod joints, and other similar connections, shortens the endurancelimit of the connections, reduces the safe load limit that can beapplied to the joints, or causes failure.

By virtue of the present invention, the stresses added to the sucker rodjoints, as a result of the pumping load applied thereto, is reducedmaterially by placing a material of a signilicantly higher modulus ofelasticity in the connection between the pin thread shoulder and the boxconnection. In the event that a coupling is used for interconnectingadjacent sections of sucker rods, the member of significantly highermodulus of elasticity would be placed between the pin thread shoulderand the opposed end of the coupling. In effect, the coupling constitutesa box portion of the threaded connection. It is found that the use of amember having a higher modulus of elasticity than the material of thepin and box between the pin shoulder and the adjacent box end lowers theadded load imposed on the pin upon each stress reversal to a significantextent, as compared to the prior threaded connection. The material usedmay, for example, be a sintered tungsten carbide formed into a washer,which will be compressed between the pin shoulder and the opposed end ofthe box upon prestressing of the threaded 3,489,445 Patented Jan. 13,1970 ICC joint. However, other materials can be used so long as themodulus of elasticity is substantially higher than that of the pin andbox portions of the threaded joint. In fact, in a general sense, if thepin and box are made of materials other than steel, such as brass, theinterposition of a cast iron or steel washer between the pin threadshoulder and the box will accomplish significant reduction in the stressin the pin of the prestressed joint resulting from the application ofload thereto. By virtue of the present invention, a much lesser load isimposed on the connection, which accomplishes applicants desirableobjectives that include increasing the endurance limit of theconnection, increasing the safe load limit that can be carried thereby,and reducing failure of the connection.

This invention possesses many other advantages, and has other purposeswhich may be made more clearly apparent from a consideration of several`forms in which it may be embodied. Such forms are shown in the drawingsaccompanying and forming part of the present specification. These formswill now be described in detail for the purpose of illustrating thegeneral principles of the invention; but it is to be understood thatsuch detailed description is not to be taken in a limiting sense, sincethe scope of the invention is best defined by the appended claims.

Referring to the drawings:

FIGURE 1 is a longitudinal sectional and elevational View of a threadedconnection embodying the invention;

FIG. 2 is a longitudinal section through a threaded connection used inits testing, and embodying a specifically different connection thandisclosed in FIG. 1; and

FIG. 3 is a longitudinal section through a prior art threadedconnection.

In FIG. 3, a typical prior art pin and box threaded connection or joint10 is disclosed for interconnecting adjacent sucker rod sections 11, 12used in a pumping oil well. Thus, one of the sucker rods 11 has a pinportion 13 projecting longitudinally from the main body of the rod,having external threads 14 and an undercut 15 extending from the threadsto a pin shoulder 16. The externally threaded portion of the pin isthreadedly received within the internal threads 17 of the box portion 18of the connection that has a counterbore 19 extending from its thread toits end 20. The threaded pin-box joint is tightened or prestressed tocause the end 20 of the box to abut the pin shoulder 16, thereby placingthe undercut portion 15 of the pin in tension and the portion 21 of thebox provided with the counterbore in compression. The tension applied tothe undercut portion 15 of the pin will exceed to a substantial extentthe pumping load to which the string of sucker rods is to be subjected,the theory, which has been proved erroneous, being that no additionalloading will be applied to the undercut portion of the pin so long asthe cyclic loading there-on is less than the initial tensile load orstress applied thereto as a result of preloading the pin and boxconnection 10.

Prestressing a sucker rod connection, such as disclosed in FIG. 3, onlypartially accomplish the desired result of reducing the deadweight loadadded to the pin stress during the pumping action. The box portion 21opposite the pin relief may be considered to be a spring undercornpression and is exerting a load through the threaded connection onthe undercut portion 15 of the pin. Thus, the load of the compressed boxportion 21 is constantly being applied to the undercut portion 15 of thepin, and when the dead load applied to the pump on the upstroke of thesucker rods is added to this force, it increases the initial tensileforce applied to the undercut portion 15 of the pin beyond that of itsprestressed value. Accordingly,

an intermittent or cyclic load is being added to the undercut portion 15of the pin, such cyclic loading imposing repeated reversals of stress onthe pin which shorten its useful life and eventually introduces failureof one of the connections in the sucker rod string.

By virtue of the present invention, the load added to the undercutportion of the pin is considerably reduced by replacing the part 21 ofthe box member opposite the pin undercut 15 with a material having ahigher modulus of elasticity than the material, such as a suitablesteel, of which the pin and box are formed. As disclosed in FIG. 1, thebox 18a hasbeen terminated substantially at the end 22 of its internalthread 17 and a washer 21a interposed between the pin shoulder 16 andthe end of the box, this washer being made of a material having a highermodulus of elasticity than the material of the pin 13 and box 18a.. Asan example, the washer 21a can be made of a sintered tungsten carbidewhich has a modulus of elasticity in excess of about 80 million. Infact, depending upon the particular sintered tungsten carbide employed,the modulus of elasticity of the washer 21a may range from about 67 toabout 97.5 million. Sintered tungsten carbide is specieally mentionedfor reasons of economy, and since it is believed to be best suited forthe purposes of the present invention. Any suitable material having amodulus of elasticity substantially exceeding that of the material ofthe pin 13 and box 18a Would also achieve the objectives of theinvention. As an example, assuming the sucker rods and their threadedpin and box portions to be made of a suitable steel having a modulus ofelasticity of about 29 million, the intervening member or washer 21a canbe made of a material having a modulus of elasticity substantially inexcess of 29 million. Thus, tungsten carbide has a modulus of elasticityof about 79 million, tungsten of about 50 million, and chromium of about36 million. In eifect, the Washer 21a of the higher modulus ofelasticity provides a higher spring rate in the region between the end22 of the box 18a and the pin shoulder 16, which -will have a lessereffect of continuing to impose its compressive force on the undercutportion 15 of the pin as loading is added to the string of sucker rods.If a material were available that was incompressible, the making of thewasher 21a therefrom would enable a system to be provided thatapproaches a 100 percent prestress condition, that is to say, theprestressing of the threaded joint to an extent greater than the maximumloading to be applied thereto during the pumping action would result inno additional loading being applied to the pin undercut 15.

Actual tests have demonstrated the signicant advantages of applicantsinterposition of a washer of relatively high modulus of elasticitybetween the pin shoulder 16 and the end 22 of the box. FIG. 2 disclosesa sucker ro-d connection on which a series of tests were conducted.Short sucker rod sections 11a, 12a were used, the sucker rods being ofthe hollow type, The adjacent sucker rods both had threaded pins 25, 26thereon interconnected by an intervening threaded coupling 30. Asdisclosed in the drawings, the coupling 30 had a lower counterbore 19bopposite the lower pin undercut 31, the lower end 20h of the couplingbearing against the pin shoulder 16 in the manner of a prior artconnection disclosed in FIG. 3; whereas, the upper end 22a of thecoupling terminated substantially at its upper internal thread, asintered tungsten carbide washer 21a bearing against its end 22a andalso against the upper pin shoulder 16, the washer being disposedopposite the upper pin undercut 32. Dimensions in inches of the variousparts are given in FIG. 2. The only variations in dimensions betweenTests A, B and C, reefrred to below, are in connection with the outsideand inside diameters of the sintered tungsten carbide washer 21a. Thesedimensions for each of Tests A, B and C are set forth in FIG. 2.

The hollow sticker rods 11n, 12a and the coupling 30 interconnecting thesame are standard parts, the dimensions given being standard. The onlyvariation in FIG. 2

4 is in connection with the upper portion of the coupling, which wasremoved, and the washer 21a substituted in its place.

In connection with Tests A, B and C specifically referred to below, twostandard hollow oil well sucker rod pony members 11a, 12a and a standardcoupling 30 were purchased off the shelf. The coupling was machined forthe substitution and insertion of the tungsten carbide washer 21areferred to above. Electronic strain gages Nos. 2 and 3 were cemented inthe pin thread bores at the thread reliefs or undercuts 32, 31,respectively. Electronic strain gage No. 1 was cemented on the outsideof one of the sucker rods 11a above the threaded joint to record thetensile load applied to the assembly.

Both joints threaded pins 25, 26 were prestressed by appropriatelythreading the pins within the coupling 30 to the extent at which straingage No. 2 showed a prestress corresponding to 44,100 p.s.i., and straingage No. 3 a prestress also corresponding to 44,100 p.s.i.

Test A In Test A, an axial load or tensile pull of 13,800 lbs. wasapplied to the sucker rods 11a, 12a, which strain gage No, 1 disclosedas being equivalent to 27,600 p.s.i. The electronic indicator connectedselectively to strain gages No. 2 and No. 3 were previously adjusted tozero with the 44,100 p.s.i. prestress applied thereto. The applied axialload of 13,800 lbs. showed an average stress at strain gage No. 2, wherethe tungsten carbide washer 21a was located, of 5,325 p.s.i., straingage No. 3 showing an average stress of 8,115 p.s.i where the tungstencarbide washer was not used, but, instead, the prior art pin and box orcoupling arrangement was employed. Therefore, there was a 34.4 percentdecrease in stress at the pin undercut portion 32 where the tungstencarbide Washer was employed, as compared to the other connection, whereit was not employed.

Test B In Test B, a dimensionally different washer 21a was used, inwhich its outside diameter was less than that employed in Test A, andits inside diameter was greater than that employed in Test A. The sameprestress was applied, namely, 44,100 p.s.i. at the pin undercuts 32, 31where strain gages No. 2 and No. 3 were located, and the same totalaxial load of 13,800 lbs. imposed on the assembly. Strain gage No. 1showed the same unit stress of 27,600 p.s.i. The average axial loadadded to the pin undercut 32, where the tungsten carbide washer 21a waslocated, was 5,600 p.s.i.; whereas, the average axial load at the pinundercut 31, where the prior art box or coupling arrangement wasprovided, was 8,000 p.s.i. Thus, the axial load added to the pinundercut 32 was about 30 percent less than the load added at the pinundercut where the prior art type of connection was employed.

Test C In Test C, the only dimensional difference, as compared with TestA, was in the slight reduction in the outside diameter of the tungstencarbide Washer 21a. Again, the two threaded joints were prestressed toprovide the prestressed load of 44,100 p.s.i. where strain gage No. 2was located, and also 44,100 p.s.i. where strain gage No. 3 was located.The entire assembly was again placed in tension by imposing a 13,800lbs. total tensile pull thereto. The axial load at the pin undercut 32where the tungsten carbide vvasher was employed increased by 4,500p.s.i., as transmitted to strain gage No. 2; whereas, the axial load atthe undercut 31 where the prior art connection was employed, namely, atstrain gage No. 3, increased by 8,000 p.s.i. Thus, strain gage No. 2where the tungsten carbide Washer is used showed the addition of a 43percent less stress to the undercut 32 than added to the other pinundercut 31.

The above actual tests disclose that the stress reversals in a threadedconnection will be significantly reduced by placing a material of asubstantially higher modulus of elasticity in the connection between thepin thread shoulder and the box connection. The term box connection isbeing used herein as including the threaded box 18a integral with thesucker rod member, as disclosed in FIG. 1, or a coupling 30 as disclosedin FIG. 2. The cyclic or intermittent applications of loading as aresult of the operation of the sucker rod string adds a much lesser loador tensile force to the pin of each sucker rod joint, enhancing theendurance limit of each threaded joint, increasing the safe load thatcan be imposed on the sucker rod string, and minimizing fatigue or othertypes of failure of the threaded joint or connection.

4I claim:

1. In a combination in which sections are secured together, suchsections being reciprocated in the performance of work and being subjectto repeated intermittent tensile loading as a result of suchreciprocation; said sections being metallic; one of said sections beingelongate and having an externally threaded pin and shoulder; a metallicbox threadedly receiving said pin and spaced axially from said shoulder;and an annular member in compression between said shoulder and box, saidannular member having a modulus of elasticity at least about 25% greaterthan the vmodulus of elasticity of said one of said sections and its pinand the modulus of elasticity of said box.

2. In a combination as defined in claim 1; said pin having an undercutportion between said shoulder and external thread, said annular membersurrounding said undercut portion.

3. In a combination as deiined in claim 1; said box, pin and said one ofsaid sections being of steel; said annular member being a hard carbide.

4. In a combination as defined in claim 3; said hard carbide beingsintered tungsten carbide.

5. In a combination as delined in claim 2; said box, pin and said one ofsaid sections being of steel; said annular -member being of sinteredItungsten carbide.

6. In a device for securing sucker rod sections together which are usedin a well bore for pumping liquids therein to the surface: a sucker rodhaving an externally threaded pin and a shoulder; a box threadedlyreceiving said pin and spaced axially from said shoulder; and an annularmember in compression between said shoulder and box, said annular memberhaving a modulus of elasticity at least about 25% greater than themodulus of elasticity of said sucker rod and its pin and the modulus ofelasticity of said box.

7. In a device as defined in claim 6; said pin, box and member incompression being prestressed when assembled together to a substantiallygreater value than the maximum pumping load to be applied to the rodsections in the Well bore.

8. In a device as defined in claim 6; said pin having an undercutportion between said shoulder and its externally threaded portion; saidannular member surrounding said undercut portion.

9. In a device as delined in claim 6; said sucker rod and its pin andsaid box being of steel; said annular member being a hard carbide.

10. In a device as delined in claim 8; said sucker rod and its pin andsaid box being of steel; said annular member being of sintered tungstencarbide.

11. In a device as defined in claim 6; said box being integral with anadjacent sticker rod.

12. In a device as dened in claim `6; said box being provided in acoupling adapted to be secured to an adjacent sucker rod.

13. In a device for securing sucker rod sections together which are usedin a well bore for pumping liquid.q therein to the surface: a steelsucker rod having an externally threaded pin, a shoulder and an undercutportion between said shoulder and externally threaded pin; a steel boxthreadedly receiving said pin and spaced axially from said shoulder; anannular sintered tungsten carbide member surrounding said undercutportion and engaging and compressed between said shoulder and box; saidpin, box and member in compression being prestressed when assembledtogether to a substantially greater value than the maximum pumping loadto be applied to the rod sections in the well bore; said undercutportion being in tension when said pin, box and member are assembledtogether.

14. In a device for securing sucker rod sections together which are usedin a well bore for pumping liquids therein to the surface: a sucker rodhaving an externally threaded pin, a shoulder and an undercut portionbetween said shoulder and externally threaded pin; a box threadedlyreceiving said pin and spaced axially from said shoulder; an annularmember surrounding said undercut portion and engaging and compressedbetween said shoulder and box, said annular member having a modulus ofelasticity at least about 25 greater than the modulus of elasticity ofsaid sucker rod and its pin and the modulus of elasticity of said box;said pin, box and annular member being prestressed when assembledtogether to a substantially greater value than the maximum pumping loadto be applied to the rod sections in the well bore; said undercutportion being in tension when said pin, box and member are assembledtogether.

15. In a device as defined in claim 14; said sucker rod and its pin andsaid box being of steel; said member in compression being a hardcarbide.

References Cited UNITED STATES PATENTS 254,477 3/1882 Hardy 285-333 X2,045,520 6/ 1936 Davison 285-333k X 2,206,166 7/1940 Dunn 285-334 X2,539,057 1/1951 Brown 285-115 X 2,955,847 10/1960 McKenna 285-286 X1,059,560 4/1913 Parker 286-117 2,834,625 5/1958 Stanley et al 286-125FOREIGN PATENTS 871,991 l/1942 France.

1,157,568 11/1963 Germany.

THOMAS F. CALLAGHAN, Primary Examiner U.S. Cl. X.R.

